A multimedia age when video signals containing a motion picture and a still image, audio signal, and computer data are integrally processed has come, and simultaneously package media such as a compact disc (CD), a digital versatile disc (DVD), and the like are popularized far and wide. Recently, attempts are vigorously carried out to apply an optical recording medium to a recording medium of a mobile phone, a digital camera, broadcasting, movie, or the like. In new generation media, this trend would be stronger.
The optical recording medium is grouped into a read-only-memory (ROM) type optical recording medium, a recordable type optical recording medium to record information only once, and a rewritable type optical recording medium to write and erase information repeatedly.
Among the optical recording media, the read-only-memory type optical recording medium has pit-shaped minute recesses that are formed in the circumferential direction and a reflective layer formed in the optical recording medium. The recordable type optical medium that would be used for the collection of backup data, broadcasting, movies, and the like has an information recording layer which may be made of organic material or inorganic material such as dye. However, when the organic material is used in forming the information recording layer, data recorded in the optical medium may not last for a long time.
In the recordable type optical recording medium, information is recorded in such way that a) the recording material is burned to generate the pits, b) volume of the recording material expands to generate the pits during the decomposition, or c) the information recording layer is melted and becomes solid again to generate new phase. To be the new generation recording medium, a very high recording density and very fast data transfer rate are required. In order to increase the recording density of information in the optical recording medium, the size of recording marks of the optical recording medium must be smaller than the of the present recording marks. Thus, wavelength of laser projected onto the optical recoding medium must be shorter than 450 nm and the numerical aperture must be also equal to or greater than 0.7. Moreover, the data transfer rate must be much faster than the present data transfer rate of 30 Mbps to 35 Mbps.
In a blue-ray disc (BD) as one of the new generation recording medium, the optical recording medium must contain a recording material in which jitter is allowed within ranges of a wavelength of 405 nm, and a recording constant linear velocity of 5.28 m/s to 10.56 m/s. Particularly, in the recordable type optical recording medium, 1) contrast between a recording mark and a space in the optical recording medium must be high, 2) the recording sensitivity is high, 3) the recorded mark must be stable (mark stability), and 4) recording characteristics including recording mark noise and the jitter must be satisfied in the BD system.
Moreover, when the laser is projected onto the optical recording medium and the recording mark is generated in the optical recording medium, laser power must not be too much higher than that which is required for the generation of the recording mark.